My theory on this comes because I happened to be looking at reviews the day before the big announcement about collections and plus, @Jiw0.
The theory: Collections are based on what guests (inadvertently for the most part) said at booking, and not on what you advertise.
The "research": On that day or two before the announcement, reviews on all listings were not chronological nor were they by country. For that brief moment in time they were grouped by "Family Trip" and "Work Trip". Weird, I thought, but as a guest, I remembered being asked, upon booking, what my trips were for. The two possible selections were Family and Work.
I noticed I had three "Family Trips" booked here, even though I have "not suitable for ages 0-12" ticked on my treehouse listing. Those three were a Girl Guide leader with her teenage charges, and two couples. Obviously in all three cases they thought, "Well, it is not work, so I guess it is family."
My treehouse is now being courted for the Family Collection in Progress. All I have to do, says the opportunities chart, is update my house rules to allow children, and then add famility amenities. Among the things I am "doing well" are three five-star reviews from families.
The room I sometimes list in the main house is much safer for toddlers - no river-with-strong-current, no 10-ft platform, no woods, no coyotes. But Family Collection is unavailable for it, as it is unavailable for yours. No one has ever booked it for a "family trip" so it is not being considered.
All you need, I think, to be considered, is to have two groups select "Family Trip" at the time they book (and then have them give you five stars). If you are keen on being part of the family collection, I suppose you could add something in the description to say "If you are booking as a family, please select 'Family Trip' as the reason for your trip."
Or you could just do as I do and make your place super dangerous.
It is not us who decides whether we are suitable for families. It is guests who are just clicking things to try to get the booking made.